I have a couple questions about amps that seem like they would be so basic, but I can't find answers on them anywhere, so here it goes:

If an amp is rated at 50W per channel, and it has 5 channels, does that mean that if you only use 2 channels that you'll be able to use 125W in each channel, or are you limited to 50W per channel no matter how many channels you use at once?

If I have a 3-way speaker and the tweeter draws 25W, the midrange draws 50W, and the sub draws 70W (for example), do I need an amp that can supply (25W+50W+70W)x2=290W per channel??? How do I calculate the power rating that I need on an amp, and how important is the power rating anyway?

The power you can get per channel varies a bit depending on how many channels are in use, but not a lot. If it's 50W/ch, then it's basically 50W/ch, baring some unusual design.

Power adds, so if you need the stated numbers for each driver, then you need the total. The numbers are not realistic however, as even a continuous watt or two into most tweeters would burn them up in no time.

That's where the rub is. Music is transient, so the average power is low and the peaks may be quite high. It would be unusual for the average power to your speakers to be greater than a few watts unless you listen really loud. The peak power might easily be hundreds of watts.

Amplifiers are best summed up by continuous power measurements, though there are power envelope techniques that are far more revealing than simple 1-number measurements. At one time the FTC required that all amplifiers be measured for RMS power in a standardized fashion, but I don't think that's the case today. Caveat emptor.

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